Cellulose is the major component of plant cell wall which constitutes 35-50% of the dry mass of plants, and thus is the most abundant renewable biomass on earth. Cellulose is a polysaccharide that is composed of glucose units linked by 1,4-β-glycosidic bonds. The highly compacted structure of cellulose microfibril contributes to biomass recalcitrance and resistance to microbial attack. Cellulose can be degraded into glucose and utilized as an energy source by numerous microorganisms, including bacteria, yeast and fungi. The complete degradation of cellulose requires several enzymes such as: endo-1,4-β-D-glucanase (endoglucanase, EC 3.2.1.4), cellobiohydrolase (EC 3.2.1.91), and β-glucosidase (EC 3.2.1.21). Among these cellulases, endoglucanase, which randomly hydrolyzes β-glycosidic bonds to cleave cellulose into smaller fragments, is the key cellulolytic enzyme. Endoglucanases are present in various microorganisms including fungi and bacteria. Based on their amino acid sequence similarities, cellulases are classified into different glycoside hydrolase (GH) families including GH 5, 6, 10, 12, 18, 45, and 74.
In recent years, the use of enzymatic hydrolysis of cellulose has been studied extensively and endoglucanases were widely applied in various industries, such as animal feed, food manufacture, textile industry and biofuel production. According to different industrial needs, cellulase is required to be suitable for different appropriate working conditions. Therefore, many scientists try to find better cellulases for industrial use by either searching new genes in nature or modifying existing enzymes. In many strategies for modifying the enzyme, protein engineering by rational design based on structural analysis is one of the major strategies for improvement of industrial enzyme. In this strategy, to increase the enzymatic activity is a key point of the industrial enzyme improvement. The higher enzyme activity represents the cost reduction of the industrial process, which further enhances the commercial profit.
Therefore, the present invention intends to analyze the enzyme structure of the cellulase for finding out the key amino acid important to the enzymatic activity and further modify the enzyme, so as to improve the enzymatic activity of the cellulase and thus increase the industrial application value of the cellulase.